Mamidi Ranganath, Gresham Kenneth S, Stelzer Julian E
Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University Cleveland, OH, USA.
Front Physiol. 2014 Dec 2;5:461. doi: 10.3389/fphys.2014.00461. eCollection 2014.
Enhanced cardiac contractile function with increased sarcomere length (SL) is, in part, mediated by a decrease in the radial distance between myosin heads and actin. The radial disposition of myosin heads relative to actin is modulated by cardiac myosin binding protein-C (cMyBP-C), suggesting that cMyBP-C contributes to the length-dependent activation (LDA) in the myocardium. However, the precise roles of cMyBP-C in modulating cardiac LDA are unclear. To determine the impact of cMyBP-C on LDA, we measured isometric force, myofilament Ca(2+)-sensitivity (pCa50) and length-dependent changes in kinetic parameters of cross-bridge (XB) relaxation (k rel), and recruitment (k df) due to rapid stretch, as well as the rate of force redevelopment (k tr) in response to a large slack-restretch maneuver in skinned ventricular multicellular preparations isolated from the hearts of wild-type (WT) and cMyBP-C knockout (KO) mice, at SL's 1.9 μm or 2.1 μm. Our results show that maximal force was not significantly different between KO and WT preparations but length-dependent increase in pCa50 was attenuated in the KO preparations. pCa50 was not significantly different between WT and KO preparations at long SL (5.82 ± 0.02 in WT vs. 5.87 ± 0.02 in KO), whereas pCa50 was significantly different between WT and KO preparations at short SL (5.71 ± 0.02 in WT vs. 5.80 ± 0.01 in KO; p < 0.05). The k tr, measured at half-maximal Ca(2+)-activation, was significantly accelerated at short SL in WT preparations (8.74 ± 0.56 s(-1) at 1.9 μm vs. 5.71 ± 0.40 s(-1) at 2.1 μm, p < 0.05). Furthermore, k rel and k df were accelerated by 32% and 50%, respectively at short SL in WT preparations. In contrast, k tr was not altered by changes in SL in KO preparations (8.03 ± 0.54 s(-1) at 1.9 μm vs. 8.90 ± 0.37 s(-1) at 2.1 μm). Similarly, KO preparations did not exhibit length-dependent changes in k rel and k df. Collectively, our data implicate cMyBP-C as an important regulator of LDA via its impact on dynamic XB behavior due to changes in SL.
肌节长度（SL）增加时心脏收缩功能增强，部分原因是肌球蛋白头部与肌动蛋白之间的径向距离减小。肌球蛋白头部相对于肌动蛋白的径向排列受心肌肌球蛋白结合蛋白C（cMyBP-C）调节，这表明cMyBP-C有助于心肌的长度依赖性激活（LDA）。然而，cMyBP-C在调节心脏LDA中的具体作用尚不清楚。为了确定cMyBP-C对LDA的影响，我们在野生型（WT）和cMyBP-C基因敲除（KO）小鼠心脏分离的皮肤心室多细胞制剂中，测量了等长力、肌丝Ca(2+)敏感性（pCa50）以及因快速拉伸导致的横桥（XB）松弛动力学参数（k rel）和募集（k df）的长度依赖性变化，以及对大的松弛-再拉伸操作的力再发展速率（k tr），SL分别为1.9μm或2.1μm。我们的结果表明，KO制剂和WT制剂之间的最大力没有显著差异，但KO制剂中pCa50的长度依赖性增加减弱。在长SL时（WT为5.82±0.02，KO为5.87±0.02），WT和KO制剂之间的pCa50没有显著差异，而在短SL时（WT为5.71±0.02，KO为5.80±0.01；p<0.05），WT和KO制剂之间的pCa50有显著差异。在半最大Ca(2+)激活时测量的k tr，在WT制剂的短SL时显著加快（1.9μm时为8.74±0.56 s(-1)，2.1μm时为5.71±0.40 s(-1)，p<0.05）。此外，在WT制剂的短SL时，k rel和k df分别加快了32%和50%。相比之下，KO制剂中的k tr不受SL变化的影响（1.9μm时为8.03±0.54 s(-1)，2.1μm时为8.90±0.37 s(-1)）。同样，KO制剂在k rel和k df中也没有表现出长度依赖性变化。总体而言，我们的数据表明cMyBP-C通过其对因SL变化导致的动态XB行为的影响，是LDA的重要调节因子。
